Mixtures of hydrogen sulfide with other gases, such as carbon dioxide and methane, are found in a number of industries. For example, mixtures of hydrogen sulfide, carbon dioxide, water and methane are found as natural gases. It is frequently necessary to remove hydrogen sulfide from gas mixtures for the purpose of purifying the gas mixture or recovering the hydrogen sulfide or both. For example, it is often necessary to purify a gaseous hydrocarbon stream to produce sweet, dry gas which will not poison certain catalysts and which will meet the usual pipeline specifications, and it is sometimes advantageous to recover the hydrogen sulfide as a source of elemental sulfur.
It is known in the prior art to absorb acid gas from an impure methane stream using a physical solvent in an absorber, to flash the acid gas-containing physical solvent stream at high pressure, to recycle the flashed gas to the absorber, to re-flash the flash liquid at low pressure, and to recycle the low pressure flash liquid to the absorber. Illustrative of this type of prior art is U.S. Pat. No. 3,594,985 to Ameen et al. It is also known in the prior art to flash the acid gas-containing physical solvent stream at intermediate pressure, to re-flash the flash liquid at low pressure and to combine the intermediate pressure and low pressure flash gases for recycling back to the absorber. Also known is a process using a tower having the intermediate third packed with rings and the upper and lower thirds as empty spaces, wherein an acid gas-containing physical solvent stream is flashed at intermediate pressure in the top third of the tower, the resulting flash liquid passes through the rings and out of the tower and is flashed at low pressure, and the low pressure flash gas is cycled into the bottom third of the tower for mixing with the intermediate pressure flash liquid.
All other prior art processes of which I am aware are deficient since they do not provide for optimum sweeping of the methane out of the acid gas-containing stream, do not provide an acid gas-containing stream with the proper concentration of carrying gas for the methane, do not provide a Claus gas for sulfur recovery that is very lean in methane, and do not provide a quantity of gas for recycling to the absorber that is less in volume but yet richer in methane, thus saving equipment cost and compression energy.